Possibility study of the partial neutron calibration for neutron flux monitors in torus devices

Abstract The absolute calibration of the detection efficiency for the total neutron yield in the whole plasma is one of the most important issues in the neutron diagnostics such as a neutron flux monitor (NFM). The possibility of a partial neutron calibration is investigated based on the in-situ neutron calibration data of the Large Helical Device (LHD), JT-60U, and other tokamaks. In the case of NFM using a 235 U fission chamber located outside of the vacuum vessel on the equatorial plane, more than 90% of the detector counts are contributed by neutrons in the toroidal angle range of −60° to +60°. The detection efficiency for the neutron source located at one toroidal position on the magnetic axis (called point efficiency) decreased exponentially with the absolute toroidal angle of the neutron source. We found that a partial calibration in the toroidal angle range of −60° to +60° combined with exponential extrapolations to -180° and +180°can estimate total detection efficiency within 5% uncertainty. In the case of point efficiency measurement at the limited number of the toroidal locations, the total detection efficiency for the whole plasma can be estimated within 4% uncertainty with the assistance of the MCNP simulation.